Resumen de Precipitation pathways and phase stability in the caso4-h2o system

María de las Mercedes Ossorio Peralta

• The Early-Stages of CaSO4 Precipitation from Solution The combination of small– and wide–angle X–ray scattering results in a powerful alliance for studying in situ and real–time the early–stages of precipitation from solution. In this study, these scattering techniques in conjunction with off–line experiments provided a benchmark to understand calcium sulphate precipitation from solution.

  As has been shown earlier, works on the early–stages of CaSO4 precipitation from solution suggested the formation of metastable phases such as bassanite and/or amorphous calcium sulphate (ACS) prior to gypsum precipitation. From the experimental results of this work, SAXS data proved that a solid phase(s) is/are formed from solution before crystalline gypsum can be detected by WAXS. The nano–sized particles identified by SAXS to appear in solution prior to the formation of gypsum did not lead to any characteristic peaks in the WAXS patterns, potentially due to the small size of these precursor species for their characterization in WAXS. Therefore, WAXS data revealed the time–resolved evolution of gypsum precipitation.

  Three CaSO4 precipitation scenarios were considered: (I) Experiments at four CaSO4 concentrations (50, 75, 100, 150 mmol·L-1 CaSO4) at 21 °C were carried out in order to monitor the early–stages of precipitation at different supersaturations and to determine the influence of supersaturation on the precipitation kinetics.

  (II) The effect of temperature on the precipitation kinetics was also studied in a series of precipitation experiments performed at different temperatures (from 4 to 40 °C) for a 50 mmol·L-1 CaSO4 solution. (III) Tracking the entire precipitation process, from early to late–stages, and determine the precipitation kinetics for gypsum forming from solution in the presence of two additives: Mg and citrate, for 50, 75, 100 and 150 mmol·L-1 CaSO4 solutions at 21 °C.

  In general, common to all the precipitation scenarios, four stages were distinguished from the analysis of time–resolved SAXS patterns: (1) Formation of sub–3 nm primary units after the creation of supersaturation.

  (2) Interactions between the primary units. These particles form 'domains' of high particle content separated from each other.

  (3) Onset of aggregation, which leads to large scattering features. Due to the fast growth of the larger aggregates, the scattering information about the internal correlations between the primary units was occulted, but it is suggested that these aggregates at short length–scales would be composed of these primary units.

  (4) Final evolution of the originally primary species at the later stages of the reaction. The formation of the scattering features was related to various rearrangements of the primary particles. The WAXS signal started developing at this stage of the reaction, and confirmed the formation of gypsum.